Prometheus: Fix interpolating minStep for range queries when incremental querying enabled (#107715)

* return calculatedMinStep in first frame of the response

* update test files

* make the custom metadata as type of map[string]any

* append calculatedMinStep to custom metadata

* fix exemplar frame checking

packages/grafana-prometheus/src/querycache/QueryCache.test.ts

@@ -646,7 +646,7 @@ describe('QueryCache: Prometheus', function () {
 });
 
 describe('findDataPointStep', () => {
-  it('should interpolate custom interval', () => {
+  it('should interpolate custom interval when there is no calculatedMinStep in response', () => {
     const mockApplyInterpolation = jest.fn().mockImplementation(() => '1m');
     const req = mockPromRequest();
     req.targets[0].interval = '$interval';
@@ -654,4 +654,20 @@ describe('findDataPointStep', () => {
     findDatapointStep(req, respFrames, mockApplyInterpolation);
     expect(mockApplyInterpolation).toBeCalledTimes(1);
   });
+
+  it('should use the calculated minStep coming in response frame', () => {
+    const mockApplyInterpolation = jest.fn().mockImplementation(() => '1m');
+    const req = mockPromRequest();
+    // Cannot be interpolated on frontend as it was calculated on backend.
+    req.targets[0].interval = '$__rate_interval';
+    const respFrames = trimmedFirstPointInPromFrames as unknown as DataFrame[];
+    const expectedMinStep = 60000;
+    respFrames[0].meta = {
+      custom: {
+        calculatedMinStep: expectedMinStep,
+      },
+    };
+    const result = findDatapointStep(req, respFrames, mockApplyInterpolation);
+    expect(result).toBe(expectedMinStep);
+  });
 });

packages/grafana-prometheus/src/querycache/QueryCache.ts

@@ -282,7 +282,9 @@ export function findDatapointStep(
 
   let dataPointStep = request.intervalMs;
   if (target?.interval) {
-    const minStepMs = rangeUtil.intervalToMs(applyInterpolation(target.interval));
+    const minStepMs =
+      respFrames[0].meta?.custom?.['calculatedMinStep'] ??
+      rangeUtil.intervalToMs(applyInterpolation(target.interval, request.scopedVars));
     if (minStepMs > request.intervalMs) {
       dataPointStep = minStepMs;
     }